In percussion or hammer drilling operations, a drill bit mounted to the lower end portion of a drill string rotates and impacts the earth in a cyclic fashion to crush, break, and loosen formation material. In such operations, the mechanism for penetrating the earthen formation is of an impacting nature, rather than shearing. The impacting and rotating hammer bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole created will have a diameter about equal to the diameter or “gage” of the drill bit.
A typical percussion drilling assembly is coupled to the lower end portion of a rotatable drill string and includes a downhole piston-cylinder assembly coupled to the hammer bit. The impact force is generated by the downhole piston-cylinder assembly and transferred to the hammer bit. During drilling operations, a pressurized or compressed fluid flows down the drill string to the percussion drilling assembly. A choke is provided to regulate the flow of the compressed fluid to the piston-cylinder assembly and the hammer bit. A fraction of the compressed fluid flows through a series of ports and passages to the piston-cylinder assembly, thereby actuating the reciprocal motion of the piston, and then is exhausted through a series of passages in the hammer bit body to the bit face. The remaining portion of the compressed fluid flows through the choke and into the series of passages in the hammer bit body to the bit face. The compressed fluid exiting the bit face serves to flush cuttings away from the bit face to the surface through the annulus between the drill string and the borehole sidewall.
In oil and gas drilling, the cost of drilling a borehole is very high, and is generally proportional to the length of time it takes to drill to the desired depth and location. The time to drill the well, in turn, is greatly affected by the number of times the drill bit or other component of the percussion drilling assembly is replaced before reaching the targeted formation. Each time a drilling assembly component is changed, the entire string of drill pipe—which may be miles long—is retrieved and removed section by section from the borehole or borehole. Once the drill string has been retrieved and the new component installed, the drilling assembly is lowered to the bottom of the borehole on the drill string, which again is constructed section by section. This process, known as a “trip” of the drill string, takes considerable time, effort and expense.
During drilling, the piston of the downhole hammer repeatedly impacts a drill bit strike face on the shank of a hammer bit. Given the magnitude of the repeated impact forces, large impact stresses occur in each member at the strike interface. In some cases, sustained impact forces may cause plastic deformation and even mechanical failure at either or both of the piston or drill bit strike faces. Additionally, fluid may become trapped between the strike faces, causing an uneven distribution of stress. So-called wash off may likewise cause mechanical failure. Replacing a damaged part due to such a failure may be costly to the operator, because the drill string may need to be tripped in order to replace the damaged part.